The present invention relates to a method for restoring the dielectric properties of a porous dielectric material whose dielectric properties have decreased as a result of contamination. In particular, the present invention provides a method for treating a porous low dielectric constant (low-k) material with a restoration fluid to displace entrapped contaminants and substantially restore the dielectric properties of the porous low-k dielectric material.
There is a continuing desire in the microelectronics industry to increase the circuit density in multilevel integrated circuit devices such as memory and logic chips to improve the operating speed and reduce power consumption. In order to continue to reduce the size of devices on integrated circuits, the requirements for preventing capacitative crosstalk between the different levels of metallization becomes increasingly important. These requirements can be summarized by the expression “RC”, whereby “R” is the resistance of the conductive line and “C” is the capacitance of the insulating dielectric interlayer. Capacitance “C” is inversely proportional to line spacing and proportional to the dielectric constant (k) of the interlayer dielectric (ILD). Such low dielectric materials (also known as, low-k dielectric materials) are desirable for use, for example, as pre-metal dielectric layers or interlevel dielectric layers. Additionally, low-k dielectric materials are being introduced to replace the conventional dielectrics in high packing density integrated circuits (ICs).
Undoped silica glass (SiO2) subsequently referred to herein as “USG,” has been long used in integrated circuits as a primary insulating material because of its relatively lower dielectric constant of approximately 4.0 compared with other inorganic materials. The industry has attempted to produce silica-based materials with lower dielectric constants by incorporating organics or other materials within the silicate lattice. For example, dielectric constants ranging from about 2.7 to about 3.5 can be achieved by incorporating terminal groups such as fluorine or methyl into the silicate lattice. These materials are typically deposited as dense films (having densities of about 1.5 g/cm3) and integrated within the IC device using process steps similar to those for forming USG films; however, there was still a need to produce materials with even lower dielectric constants because transistors need to be placed even closer together to make higher-speed chips. This necessitates that the insulating layer become even thinner, thus leading to charge build-up and crosstalk, which adversely affects the performance of the chip.
Because the dielectric constant of air is nominally 1.0, it was postulated that incorporation of air into the dense material to form pores would result in reducing the dielectric constant as well as the density of the material. For a porous film, the dielectric constant is, thus, a combination of air and the inherent dielectric constant of the material comprising the dense phase. Such combination has the potential to drive the dielectric constant significantly below two.
One method for introducing porosity into a film employs thermal annealing to decompose at least a portion of the film's components thereby creating pores. In the annealing step, or curing step, the film is typically heated to decompose and/or remove volatile components and substantially cross-link the film. U.S. Pat. No. 6,312,793, for example, describes a multiphase material having a first phase consisting essentially of silica (Si), carbon (C), oxygen (O), and hydrogen (H), a second phase consisting of carbon and hydrogen. The material is heated to a temperature of at least 300 degrees Celsius (° C.) for at least 15 minutes to induce removal of the hydrocarbon material.
Another example is found in published International Patent Application WO 00/02241, which describes heating an alkoxysilane material at a temperature from 100° C. to 400° C. for a time of about 1 minute to about 10 minutes to induce formation of pores by removing the solvent contained therein.
Yet another example is found in published International Patent Application WO 02/07191, which describes heating a silica zeolite thin film to a temperature range of 350° C. to 550° C. for an unspecified amount of time to induce adsorbed material to leave the zeolite framework thereby lowering the dielectric constant.
Although the incorporation of pores into dielectric layers is beneficial with respect to the material's insulating properties, there are also drawbacks. One drawback, for example, is that problems can arise when process chemicals and other contaminants penetrate and become entrapped in the pores. When contaminants become entrapped in the pores, the dielectric constant of the material typically increases, thereby reducing the material's dielectric performance.
Typical sources for contamination of porous dielectric materials include, for example, any process step in which a substrate is exposed to liquid process chemicals such as, for example, those used in wet etching, resist stripping, chemical mechanical polishing (CMP), post-CMP cleaning, photolithographic development, etc. Such process chemicals are likely to penetrate the pores in the porous substrate and remain entrapped as a contaminant. Each type of contaminant has a potential for interfering with fabrication processes and adversely affecting the device's dielectric performance such as, for example, increasing the dielectric constant, increasing the leakage current, and reducing the dielectric breakdown voltage. As total elimination of contaminants from process environment is not possible, elaborate methods of semiconductor surface cleaning and conditioning are employed throughout the device manufacturing sequence.
For example, use of a non-porous silicon carbide capping layer on top of the porous dielectric layer was proposed as a means to avoid the direct contact between the porous dielectric layer and the liquid process chemicals. Even if such layer is used, however, there would still be a risk of chemical penetration in the porous dielectric layer either through edges of the films or through a loss of capping layer resulting from mechanical stresses experienced during processing such as, for example, those imparted by CMP.
An apparatus and method of removing contaminants from the voids and pores of a porous dielectric material has also been proposed. The method includes an apparatus comprising a wet-cleaning chamber, a supercritical drying chamber, and a substrate transferring chamber which transfers a substrate to and from the wet-cleaning chamber and supercritical drying chamber. The substrate is cleaned and dried in the wet-cleaning chamber to remove any visible residues and liquids and then subjected to a drying or “microscopic drying” step. This can be done using a supercritical drying chamber using a supercritical fluid under pressure or a low-pressure chamber at high temperatures to remove residual liquids from the voids and pores in the substrate. This method, however, requires special equipment that adds expense and time to a process that manufacturers are trying to reduce costs and streamline operations.
Accordingly, there is a need in the art to provide an improved, cost effective method for restoring the dielectric properties of a contaminated low-k dielectric film that does not suffer from the above-mentions drawbacks.